Measuring Low-Level Signals Using Attenuation, Video
Bandwidth, and Video Averaging
Spectrum analyzer sensitivity is the ability to measure low-level signals. It is limited by the
noise generated inside the spectrum analyzer. The spectrum analyzer input attenuator and
bandwidth settings affect the sensitivity by changing the signal-to-noise ratio. The attenuator
affects the level of a signal passing through the instrument, whereas the bandwidth affects the
level of internal noise without affecting the signal. In the first two examples in this section, the
attenuator and bandwidth settings are adjusted to view low-level signals.
If, after adjusting the attenuation and resolution bandwidth, a signal is still near the noise,
visibility can be improved by using the video-bandwidth and video-averaging functions, as
demonstrated in the third and fourth examples.
Example: If a signal is very close to the noise floor, reducing input attenuation brings the
signal out of the noise. Reducing the attenuation to 0
dB
maximizes signal power in the
spectrum analyzer.
Note
The total power of all input signals at the spectrum analyzer input must not
exceed the maximum power level for the spectrum analyzer.
1. Connect an antenna to the spectrum analyzer input. Press
Cm).
2. Reduce the frequency range to view a low-level signal of interest. For example, narrow the
frequency span from 88 MHz to 108 MHz by pressing
[FREQUENCY],
START FREQ , 88 (MHz),
STOP FREQ , 108
IRnHz).
3. Place a marker on the low-level signal of interest. Press
(MKR)
and use the knob to position
the marker at the signal’s peak.
4.
Place the signal at center frequency by pressing
(MKR]
then MARKER -CF.
5. Reduce the span to 10 MHz. Press (SPAN), and then use the step-down key
(m).
See Figure 3-16.
4
MKR
164 93
MHL
REF
0
dRrn
AT
10
‘38
-57
12
d&r
--
PEAK
LOG
10
-
dB/
SPAN
in
00
MHZ
Figure 3-16. Low-Level Signal
Making Basic Measurements
3-l
5
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